The present invention relates to a probe assembly and a method for measuring resistivity of a soil. The probe assembly is adapted to be connected to a measurement apparatus capable of generating a constant DC current signal into two electric leads.
Since the electrical resistivity of the soil can change with moisture content and temperature, and since soil is used as electrical isolating material, the measurement of soil resistivity is therefore important.
Known in the art, there is the Schlumberger method which is one of the principal means for measuring soil electrical resistivity. This method determines the voltage drop between two electrodes, caused by a current flow between a second pair of electrodes. The first pair of electrodes is mounted between the second pair of electrodes. By measuring the resistance between the first pair of electrodes, and assuming hemispherical current flow in homogeneous soil, it is possible to measure the resistivity which is related to the resistance that is measured. In order to minimize errors from polarization of the soil, or existing earth currents, the current source is usually AC or chopped DC current at a frequency which is neither 60 Hz nor a harmonic of 60 Hz. In urban areas, where the conditions around a structure can be markedly different from those in the nearby soil, the present method is not appropriate.
It has been found that the Schlumberger method can be relatively efficient for measuring the resistivity of the soil at certain depths in the soil but is not efficient for measuring the resistivity of a thin layer of the soil located at or nearby the surface. Also it has been found that the measurement of the soil with the Schlumberger method cannot be done in a continuous manner because of polarization of the soil that is produced.
It is an object of the present invention to provide a probe assembly and a method by which it is possible to measure the resistivity of a thin layer of the soil located at or nearby the surface.
It is also an object of the present invention to provide a probe assembly and a method by which the resistivity of the soil can be measured without producing polarization of the soil.